This invention is directed to an aqueous dispersed polymeric binder comprising a grafted terpolymer containing copolymerized units of glycidyl acrylate, epoxy resin, diphenol such as bisphenol, grafted with copolymerized ethylenic monomers, where the binder polymer includes self-contained copolymerized wetting agent groups to facilitate adhesion of the binder to unclean substrate surfaces such as steel surfaces containing dirt and/or oil contaminants. Ordinarily, wetting agents are separate, external additives and comprise a hydrophobic and hydrophilic chain or group to obtain compatibility with the water dispersed binder as well as provide wetting properties to oily surfaces. Such additives ordinarily comprise a short aliphatic chain with a polar group such as a carboxyl group.
In accordance with this invention, the external additive wetting agent can be omitted by building certain wetting agent groups into the polymer structure. The resulting polymeric binder is particularly suitable for the interior surfaces of aluminum or steel containers and cans used for beer and beverages.
Epoxy resins are particularly desirable for use in surface coating materials as a vehicle or polymeric binder for pigments, fillers, and other additives whereby epoxy resins advantageously provide toughness, flexibility, adhesion, and chemical resistance. Water-dispersed coating compositions containing epoxy resins are highly desirable for can coating compositions. Coatings for soft drink and beer cans, for instance, are critical due to taste sensitivity wherein such sanitary can coatings must not alter the product taste of canned beverages. Taste problems can occur in a variety of ways such as by leaching of coating components into the beverage, or by adsorption of flavor by the coating, or sometimes by chemical reaction, or by some combination thereof. In commonly assigned U.S. Pat. No. 4,212,781, a process is disclosed for modifying epoxy resin by reacting the epoxy resin with addition polymerizable ethylenic monomer in the presence of at least 3% by weight of benzoyl peroxide (or the free radical initiating equivalent thereof) based on monomer at a suitable reaction temperature. This reaction produced a reaction mixture containing an in-situ formed blend of resinous materials comprising an epoxy-acrylic copolymer mixture containing epoxy resin, graft epoxy-acrylic polymer, and associatively-formed ungrafted addition polymer. The in-situ polymerized monomers include acid functional monomers to provide acid functionality in the reaction mixture sufficiently high to effect stable dispersion of the resulting reaction product in a basic aqueous medium. Similarly, commonly assigned U.S. Pat. No. 4,522,961 is directed to a mixture of polymers comprising a self-curing water dispersed or emulsion polymer, an epoxy-acrylic graft copolymer, and a phosphate additive, where self-curing water dispersed polymers were produced by polymerizing ethylenic monomers, including functional carboxyl, hydroxyl, amine or amide monomers in combination with alkylol acrylamide monomers, and thereafter blending the water-dispersed polymer with an epoxy-acrylic copolymer and preferably with a phosphated additive. Related commonly assigned patents are U.S. Pat. Nos. 4,285,847 and 4,399,241.
It now has been found that such compositions can be improved by incorporating wetting agent functional groups into the binder polymer structure itself whereby an additive wetting agent is avoided, VOC (volatile organic compounds) is reduced, and good wetting and good adhesion can be obtained with metal substrates without causing blistering problems. These and other advantages of this invention will become more apparent by referring to the detailed description and illustrative examples.